


Deeds of Darkness

by Ciala



Category: Undertale (Video Game)
Genre: Angst, Angst with a Happy Ending /Semi-Happy Ending, Gaster vs Dadster, Gen, Major Character Injury, Major Character Meltdown, Sans and Papyrus are little kids during this and get experiemented on, baby bones, bad side effects of experiements, experiment au, time line hopping Gaster, very bad
Language: English
Status: In-Progress
Published: 2015-12-21
Updated: 2017-09-23
Packaged: 2018-05-08 04:40:29
Rating: Not Rated
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 2
Words: 3,775
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/5483798
Author URL: https://archiveofourown.org/users/Ciala/pseuds/Ciala
Summary: <blockquote class="userstuff">
              <p>When an injection of Determination on a test subject has horrifying results, W. D. Gaster turns to his timeline machine in order to hop timelines to get a new specimen....</p>
            </blockquote>





	1. Meltdown

**Author's Note:**

  * For [13thcat](https://archiveofourown.org/gifts?recipient=13thcat).



> Warning: fiction contains experimentations of young Sans, and its effects on him.  
> \----  
> Idea of Gaster vs Dadster: http://anic-mj.tumblr.com/  
> Experiment AU: (no clue, do you guys know?)
> 
> {With the emergence of Gaster, and the knowledge of Sans special powers, I had speculated that maybe something was done to Sans to get his powers.}
> 
> I know @zarla-s has the handplates au, but I don’t know who did the experimentation au.
> 
> I’m completely knew to writing for Gaster and knowing things about Gaster( I know some things but not a lot. )
> 
> If anyone sees something that is related to another AU, let me know to give credit. Most of this stuff is off the top of my head.

Sans is lying on a table, his small childish form struggles against the straps that hold him there.

There’s a tall figure draped in a large white lab coat near Sans. He stands next to a machine. The machine beeps and a drawer opens. Mists wafts out from the opening, and the figure takes out a needle that is filled with luminous red liquid. He turns toward Sans, and looks directly in his eyes.

Sans freezes in a moment of panic. Something about the figures eyes sends chills down his spine. They are large pools of blackness with small dots for pupils, which other than being a little creepy are not much too look at. But the feeling he got from them, it was indescribable. And then the cracks that they have…it just adds to the effect. He smiles sinisterly at Sans.

That breaks Sans from his stupor, and he renews his attempts to escape.

The figure approaches Sans, and frowns severely at him. 

“Stay still,” he says.

The cold severity in his tone made Sans instantly stop.

He injects half of it into Sans’s humerus bone, and then withdrawals the needle.

The bone he injected it into burns and stings. It feels like it’s on fire. Sans wiggles around a bit as the pain spreads to the other bones in his arm.

“S—.”

Sans only hears part of his name. The pain distracting him from the figure as the feeling now spreads past his arm and into the rest of his body.

“S—!” The sharpness of the figure’s voice grabs his attention. “Now hold still, this will hurt a lot, and if you move during it, you may not recover.”

 _W…what?_ Sans thinks, frantically. _What does that mean!?_

The scientist hovers the half-filled syringe above Sans soul for a fleeting second, and then plunges the needle into his soul.

It was a sharp sensation but cold too. Sans mind blanks out momentarily, but he is soon brought back to his senses as a stabbing pain sears through his soul. A harsh fiery feeling engulfs him. He cries out, writhing in agony. He struggles against his bindings, but they hold fast. He feels something foamy coming out of his mouth, and his body goes into convulsions. Then all goes dark…

Sans jerks awake, gasping for breath. The bones in his body ache, and his head throbs with a dull ache. He looks around, and calms down a bit. The room he is in a sterile room with metal flooring and white walls with large windows on of its sides. He’s on a pile of blankets next to his sleeping brother, who is year younger than him.

He’s not in that room with that person. What do they call him? Something Gaster? Not that it matters, they are never to address him by his name but only as Sir. But Sans would still call him that mentally, as a sort of silent rebellion against him.

He sighs deeply and tries to collect himself. That was a little over a month ago, but the events still shake him to his core. And not just figuratively. That injection did something to him. Ever since that shot, he’s had trouble getting a full night’s rest. He got shuddering fits where his bones would rattle nearly all the time, and not just on the outside, but the inside too. Those fits lessened in occurrence and severity, but they still occur ever so often.

Fortunately, Papyrus never saw him have those attacks, and Sans never wants him to. It would make him worry, and up till now they only really occurred when he was with Gaster. It made it nearly impossible for him to concentrate and able to perform the tasks Gaster had wanted him to do. The scientist always got angry when that happened, and told him that if he didn’t do better he could always use his brother instead of him.

That was always his trump card, and it won all the time. Sans will do anything to protect his dear brother from sharing his fate, and Gaster knows it. So Sans would always pour all his concentration and strength into fulfilling Gaster’s orders. But sometimes that wasn’t enough, and Gaster would punish him…

Sans shakes his head. He doesn’t want to remember that. He tries to think of more pleasant things. Like being with his brother, making up stories for him, and seeing him smile.

Sans’s bones start to tremble.

 _Darn it,_ he thinks, wrapping his arms around himself in an attempt to stop the shaking. _Why do I have to have another attack right now? What if Papyrus sees me?_

Papyrus…

He looks over at him, a gentle expression enfolds on his face. Papyrus, who is just a bit younger than him, is sleeping peacefully on the makeshift nest.

The vibrating of his bones gets worse, and Sans closes his eyes tightly. He takes in deep breathes.

_Please, let it end soon… Please…_

Twenty minutes later, the shaking of his bones stops as does his headache and the dull ache in his bones.

Sighing, he curls up in the blankets. He plans on going back to sleep, but there’s a buzzing overhead. The sound gets louder, and Papyrus’s face scrunches up in annoyance.

Sans eyes widen in horror as he remembers what that noise means.

_He’s coming. Gaster is coming._

Sans sits up and dread fills his young heart. His body stiffens as hard as a board as he sees the silhouette of the scientist pass by the windows of his room.

He feels a hand wrap around his leg, and he jumps a bit. He looks down and sees a trembling Papyrus glancing at the window and then to him.

Sans gives his brother a reassuring hug. He’s unsure how effective it is since he too is shaking, this time from fear.

Gaster stops in front of door, and he waves his hand in front of it.

The metal door opens silently.

“S/4NS.” The doctor calls.

Sans shudders at his technical name. Before he goes over to Gaster, he looks at Papyrus for a brief second. He instantly regrets it. The look of fear and sadness on his brother’s face sends pangs of pain deep into his heart. Sans smiles weakly at him, and then walks over to Gaster.

As Sans steps out of the doorway, the door instantly shuts.

“Follow me.”

Sans silently obeys. His heart pounds in his chest, and the bones in his legs feel weak and wobbly.

 _Why? Why does this time feel so much different from the others so similar to it?_ He wonders. _Maybe it was just because I’m still effected by the shot I had a month ago? The one I just had a dream about._

Sans shudders at the thought of it. He instantly looks up at the scientist, hoping he doesn’t see his fear. Gaster doesn’t like them to show any signs of weakness. They are punished whenever they do show signs of it.

However, the scientist doesn’t notice and keeps walking down the barren corridor.

Sans decides to turn his thoughts to other things to distract himself from that time. He looks around at his surroundings. The walls are gray and blank with no distinguishing marks on them, the floor is a white tile.

_Everything in this place seems so lifeless… lifeless…_

He shivers at the thought, and instantly grits his teeth. He does his best to tense up his bones so he doesn’t show how worried he is to the scientist. He looks up at him again to make sure that he isn’t in trouble.

But the doctor is still not paying any attention to him. Instead he is looking at a clipboard with some weird words in it, and mutters about deadlines and something about Determination at 10 times the level of the previous injection.

 _What does that mean?_ Sans wonders.

He follows the tall scientist to another room, and his heart instantly sinks. There in the middle of the room is a table with straps on it, and beside it is a machine.

 _No…_ Sans thinks as fear grips his soul with in its icy clutches. _No…This is just like that one time…_

.  
.  
.  
.  
.  
.

Gaster sits on a stool near the table, observing the effects of the booster shot of Determination on his test subject. He frowns, and turns toward the machine and begins typing into it.

> Specimen G-S/4NS is exhibiting increased signs of distress after booster shot. Convulsions and foaming of the mouth are more severe than the previous time. Specimen is unresponsive to my voice or instructions. His heart rate is erratic, and he is sweating profusely. I will be moving him to an observation chamber and will attach him to a machine to monitor his cond- 

A sudden noise from the table Sans is on catches Gaster’s attention, interrupting his note taking. He swirls around and nearly gasps out in terror.

Sans’s small body is starting to melt away. Gaster rushes into a nearby room and grabs a small metal box. He runs over to the table and touches the cube to it. The cube automatically merges with the table and forms a containment unit for Sans, able to catch and retain his melting form.

Sans reaches up an arm that drips down into his body. His eyes wide in horror. “Paaapyyruss….” He moans.

Gaster watches in horror and fascination as Sans body becomes a glob. It calls out for his brother.

As in response, Papyrus from far down the hall, starts calling for Sans. His voice gets higher and higher and it starts to drive Gaster crazy.

Gaster closes the door, and quickly inputs the Sans’s changes into the machine. He presses a button which summons some lackeys to take Sans to the observation room and hook up machines to his melted form.

As he watches them take Sans away, he realizes that his most promising test subject has failed him, and all the hard work he had in him has gone to waste.

 _This is bad,_ he thinks, _S/4NS is a far better test subject than his brother. While P/4PYR05, is stronger, he has completely weaker disposition and a more sensitive mental state. If only he had been an identical twin to S/4NS, then all he would have to do was adjust the levels of Determination, and all his hard work would not have been lost. After all, training and molding the young skeleton isn’t difficult, it was just having the right specimen for the serum…Well, no even identical twins are different... There would still be problems... He sighs. If only he just had another S/4NS…_

His eyes widen and a smile creeps on his face.

_That’s it! Another S/4NS…. I have a machine that might help him with that!_

He dashes over to into another room with odd machines and monitors next to a strange archway.

He types something in the machine. He sees different wavelengths in the one monitor, and in the other monitors he sees skeletons of different shapes, sizes, and ages. They were all of Sans and Papyrus.

_My timeline machine! Gaster thinks, I made it watch over different time lines to learn from the errors of my other selves, and make important discoveries… but I think that with the right amount of tweaking, I can enter into one…and then I can just take one of the S/4NS’s that are there…but it needs to be around the same age, size and appearance of the one that melted… That way no one will be the wiser, I won’t get into trouble for melting my speicimen, and his younger brother will think he has gotten his brother back and thus will shut up and stop calling for him._

_Now I just need to find the right one…_


	2. Encroaching Darkness

**Summary for the Chapter:**

> Sans (as a babybones) is having nightmares and Gaster (Dadster) is trying to cope with that and his job. Grillby offer his help.

Sans bounds happily down the busy streets. His youthful vigor giving him an abundance of energy to track down his prey, the ice cream vendor. As he sees the Nice Cream Cart, his eyes sparkle with excitement. 

“Sans, wait!” His dad calls. He’s quite aways from his son. He hurries toward him, however he does not move as fast as he would like to. Carrying a little baby bones Papyrus slows him down considerably. 

Hearing his dad call him, Sans looks back and sees his family. He stops and wonders if he should stay or if he should go. He glances at the cart, and then back to his family. With a loud sigh, he decides to wait for them. It’s an antsy wait.  

Sans bounces around eagerly. “Hurry, dad! Hurry!” He points the Nice Cream Cart.

“Patience, Sans,” Gaster says between breaths, as he finally catches up to Sans. “The Nice Cream will still be there when we get there.”

Sans tugs Gaster’s sleeve, grabs it, and hurries them all over to the cart. 

As they do so, the bustling streets became deadly quiet. Sans looks around to see why, and he sees that all the color drained off all the residents. They were all gray with black and scraggly eyes. They all stop what they are doing, and turn towards Sans. They open up their shadowy jagged mouths a low hissing sound comes out. 

Sans looked up at the nice cream vendor. Her skin is also as gray as dust and her eyes were dark squiggles. She grins at Sans, revealing pointed teeth. Squeaking in fear, and backs away from her. He looks up at his dad to tell him that is something wrong when his dad starts to dissolve away right before his eyes. He goes to look for his brother, but he is no where to be seen.

The citizens lumber toward Sans. 

“Sans! Sans….” The monsters spookily moan. They reach up for him, and as they do so their arms stretch out to reach him.

“Nooooo!” Sans cries as he is grabbed.

 

He bolts up in bed, wide awake. Tears stream down his face, and a blue glow comes from his eyes. He’s confused. Where is he? This isn’t the alleyway or the streets…. 

“Sans…” comes his dad’s voice. “It’s alright, son… I’m here. You’re safe.”

Sans looks over at him. “Dad…?” His mind starts to clear as he remembers he’s at his home. The Toys and a lamp he unintentionally lifted with his magic, softly drop to the floor as he calms down. 

“That’s right,” his dad says soothingly, as he gives him a reassuring hug. He sighs with relief as the signs of worry of his face start to fade. “Everything is ok… You’re safe and at home. You just had a nightmare.”

Sans hugs his dad tight. “Dad! It was so scary!” He sniffles.

“I’m sure it was, but it’s all gone now. No more scarries.” He pats Sans head. “How about you try to go back to sleep?”

Yawning, Sans reluctantly agrees and curls up in under his sheets.  Fortunately no more bad dreams would come to him this night.

* * *

 

Later that day, at his work, Gaster moves about sluggishly from station to station. 

“Sir, you look dead. Should you even be at work today?” asks one of Gaster’s employees.

Yawning, Gaster nods. “My son has just had a nightmare last night… so I didn’t get a lot of sleep.”

“Was it Sans?” asks another employee.

Gaster nods.

“But he’s had one every day of the week then, right? That can’t be good for either of you.” Says a third employee. 

“Yes and yes...” Gaster sighs. “It’s taxing on both of us.” 

His employees gather around to offer words of support.  
“That’s rough….”  
“What a trouper…”  
“That’s our boss for ya.”  
“You’re a rock, Dr. G.”

  
As the words leave the last one, Grillby comes in with a cart of food. “Catering to the rescue.” He smiles.

The workers perk up and rush over to him.

“Alright, lunch time!”

“I’m famished!”

As the employees quickly grab some food, Gaster turns toward Grillby, a confused expression on his face.

 “I didn’t order catering…”

“My treat to all of you. Especially you, Gaster. I know, but given how hard your week has been what with having to be up with Sans every night, I thought you could use it.”

Gaster smiles. “Thank you.”

“Wait, so you know about the nightmare problems too?” asks an employee. 

Grillby nods. “Gaster and I are good friends,” 

As Gaster goes to pick up a sandwich his phone beeps. He takes it out, looks at it and just sighs.

“What is it?” Grillby asks.

“The king requests if I can be here again tonight to go over some plans for the Core. I normally wouldn’t mind, but….”

“Right. You haven’t been getting enough sleep. Why don’t you call of early today so you can be ready for the meeting tonight?”

Gaster thinks for a bit, trying to decide if he should. “Hmm… Yes… I suppose I should. I just have to find a sitter for the kids for tonight.”

“I’ll do it.” Grillby offers.

Gaster shakes his head. “Grillby, you’ve done enough already with this free lunch...”

“No more protesting, Gaster. After the week you’ve had, you need as much help as you can get, and I would like to you help you where I can.”

“Thank you, I really do appreciate it.”

“No problem. Now why don’t you go get some rest and spend some time with your kids. I bet they would love that.”

 

Gaster nods, and heads off home. Since the kids will still be at home for a few hours, he decides to nap for a bit.  

“Now, kids, I have to work tonight.” They whine about that. “But that is why I am having Grillby watch over your tonight. So if you need anything, he will be there to help you.”

“Yay, Gibs!” Says Papyrus, clapping his hands.

Sans smiles at the mention of the bar tender, though he still seems tense.

_He still seems bothered by his nightmare,_ Gaster thinks, as he looks at his son.  _I need to do something for him, but what…_ He hears the jingle of a near by Nice Cream Cart. 

“Say, how would you two like some ice cream?” 

“Yaaasss!!!!” Papyrus waves his arms.

Sans eagerly nods. 

“Alright, let’s go then!” Gaster picks up Papyrus, who squeals with delight. 

With that Sans dashes off to find the Nice Cream Cart.

“Sans, not so fast!” His father says. Smiling, he watches his oldest weave past people on the bustling side road. 

Sans’s laughter rises above the murmurings of the other citizens. 

Hearing it brings happiness and relief to Gaster, who had been silently fretting about Sans and his nightmares.

Sans easily tracks down the jingle of the cart and rushes over to the vendor.

“Well, hello there, Sans, would you like menu?” A purple bunny asks Sans.

  
“Yes peas!” Sans nods his head vigorously.

The lady chuckles, and hands Sans a picture menu of the ice cream. Sans eagerly takes it and begins to look at all the options.

His father finally catches up, and the vendor turns to him. 

“Ah, Dr. Gaster, it’s a pleasure to see you again,”  The bunny beams. “Here’s a little menu for that cutie you have with you.” She hands Gaster another menu, and Papyrus coos at her compliment.

Gaster chuckles. “Why, thank you. And yes, It’s been a few days since we’ve been here. Thought I’d let the kids have a treat for being so good.”

“Ooh oooh!” Sans exclaims as he sees something on the menu. 

“Do you see something you want, Sans?” His father asks.

Sans nods his head so fast he looks like a bobble doll. “Want this!” He points to a star shaped ice cream bar.

“Heh, I should have known,” Gaster smiles at his son, and pats him on the head with his free hand. “Alright. one Star Cream and… What do you want Papyrus?” He asks as he turns to Papyrus, who is holding his menu and looking very seriously at the options. 

“Hm….Dis!” Papyrus points to an echo flower shaped sherbet. 

“And an Sherbecho.” Gaster says, as he takes the menu from him. 

“Yay!!!” Papyrus exclaims.

Gaster chuckles, and gives his son a soft head boop of affection.

The bunny lady smiles at the scene. “And what would you like, doctor?”

“I’ll have a chocolate and vanilla twist cone.” 

“Alright.”

As Gaster pays, they find a bench to sit down at and enjoy their nice cream. 

Sans is chomping at his, while Papyrus is delicately eating his treat.

Gaster takes a few bites of his, and then turns to Sans.

“So… Sans are you feeling better?”

Sans stops eating his ice cream and just stares at it silently, as if the question brought back some unplesant memories. A few seconds pass, and he blinks. All traces of unhappiness leave his face.

“Yea! It’s good!” He turns to his ice cream with a determined look and takes a big chomp at it.

 

After ice cream, they head back home and wash up for dinner. 

Gaster makes them spaghetti, and serves it to them in little bowls and forks. When they are done eating, he put the dishes in the sink, and put them in their pajamas. 

He tucks Papyrus in first. He sits on the edge of his bed, and holds Sans comfortingly. He summoned some floating hands that went over and picked up a book, and brought it over to him. 

As he reads them their bedtime story, he notices that they are both getting sleepy. A few minutes pass, and Papyrus is out cold. Gaster glances at Sans and sees that he is about to nod off. 

He carries Sans into his room and places him gently on his bed, and tucks him in.

“Mmmmm noo…” Sans sleepily protests. “No go…”

Gaster pats him on the head. “Shh… It’s ok. Daddy’s got to go to work. But Uncle Grillby will be here to watch over you and to keep you and your brother safe. Alright?”

Sans nods. “Aright.” He yawns and drifts off to sleep. 

Gaster watches his son sleep for a bit and then walks out to the hall. He sees Grillby by the door. 

Grillby grins and pats the doctor on his shoulder. “I’ll keep watch over them so you can work without having to worry about them. If Sans has a nightmare, I’ll calm him.”

“Thanks, Grillby,” Gaster nods. “You’re a life saver.” 

With that, Gaster heads down the stairs and leaves for work.


End file.
